Most vacuum electron tubes employ at least one thermionic oxide cathode as a source of electrons. A typical cathode comprises a nickel metal substrate, a layer consisting essentially of barium oxide and one or more other alkaline earth oxides on one surface of the substrate, and means for maintaining the operating temperature of the substrate at about 950.degree. to 1100.degree. K. opposite the other surface. The substrate contains minor amounts of reducing agents which progressively migrate at different rates into the oxide layer at the operating temperature and reduce the barium oxide in the oxide layer to barium metal. The barium metal produces a low work function surface on the oxide layer for the efficient emission of electrons at the operating temperature. An article by A. M. Bounds et al. entitled, "Nickel Alloys for Oxide-Coated Cathodes," Proceedings of the I.R.E., 39 788-799 (1951), discloses that the commonly-used reducing agents in the substrate are elemental aluminum, carbon, magnesium, manganese, silicon, titanium and tungsten.
Minor amounts of elemental silicon are alloyed with nickel in the substrates of all commercial oxide cathodes, even though a resistive interfacial layer of barium orthosilicate is known to form between the substrate and the oxide layer during the operation of the cathode. To limit the formation of this interfacial layer and thereby extend the life of the cathode, the concentration of silicon in the substrate is usually less than 0.1 weight percent and never more than 0.25 weight percent. The other reducing agents mentioned above are similarly limited in concentrations in the substrate.
Chromium metal, which has been reported as a reducing agent, is never intentionally present in significant quantities in the substrate because it is reported to form a heavy black interfacial layer between the substrate and the oxide layer which interferes with the operation of the cathode, and because it is believed that chromium metal sublimes too rapidly at the operating temperatures of oxide cathodes to be practical. U.S. Pat. No. 4,370,588 issued Jan. 25, 1983 to K. Takahashi also points out that chromium that is diffused into the oxide layer will shorten the emissive life of the cathode.